This invention relates to optical fibre amplifiers, such as erbium doped fibre amplifiers. In such an amplifier pump power at a wavelength .lambda..sub.p is launched into the amplifier fibre in order to produce the requisite population inversion necessary to promote amplification of the signal power at a wavelength .lambda..sub.s also caused to propagate along the amplifier fibre. The pump power is combined with the signal power by some form of wavelength multiplexer, which may for instance take the form of a dichroic mirror or a 2.times.2 tapered fused fibre coupler. An example of such use of 2.times.2 couplers is for instance to be found in the amplifier described in GB-A 2 230 912 with particular reference to its FIG. 5. That particular amplifier employs bidrectional pumping, and so a wavelength multiplexing 2.times.2 coupler is employed upstream of the amplifying fibre for the co-pumping, and a further wavelength multiplexing 2.times.2 coupler is employed downstream of it for the counter-pumping.
The fibre from which the two wavelength multiplexing 2.times.2 couplers is made is typically conventional single mode transmission type (non-amplifying) fibre, whereas the amplifier is different because it is required to incorporate the material that renders it optically amplifying. If the two types of fibre happen to have substantially the same refractive index profile, a relatively low loss optical coupling between each end of the amplifier and an associated one of the ports of the two 2.times.2 couplers can in principle be made by simple fusion butt splices. However, high efficiency optically amplifying fibre typically has a refractive index profile significantly different from that of conventional single mode optical communications fibre. As a result of this difference, the two types of fibre have significantly different spot sizes, and this means that a simple fusion splice of the one type of fibre to the other gives rise to a splice loss that is inconveniently high for some applications. In some circumstances the amount of this loss can be somewhat reduced by heating the splice so as to promote localised diffusion of the core region of the optically amplifying fibre into its cladding so as to produce a localised reduction in its spot size. This is for instance described by A. Wada et al in a paper entitled `High Efficiency Erbium-Doped Fiber Amplifier using Mode Field Diameter Adjusting Technique` given at the second topical meeting on optical amplifiers held in July 1991 at Snowmass Colo. (pp 258/FD3-1 to FD3-4/261). This paper explains that diffusion produced by localised heating in the region of the splice has no effect in reducing the effects of spot size mismatch if the diffusion constants of the two fibres are substantially matched, and that in consequence the technique can be effective only if the smaller spot sixe amplifier has a significantly larger diffusion constant than the transmission type fibre to which it is being spliced.